Stereophonic signal reproducing apparatus

ABSTRACT

In a stereophonic signal reproducing apparatus, there are provided left and right loudspeaker systems for reproducing sounds in response to oppositely phased difference signals (L-R) and (R-L) that are supplied thereto, a central loudspeaker system disposed between the left and right loudspeaker systems for reproducing sounds in response to a sum signal (L+R) supplied thereto, and a frequency characteristic compensating circuit operable upon the oppositely phased difference signals to vary the frequency response of a signal reproduced from the difference signals such that the reproduced signal strength is relatively high in a low frequency band but relatively low in a high frequency band as compared to the frequency dependent signal strength of the sum signal (L+R). The stereophonic signal reproducing apparatus further has an auxiliary output terminal to which a sound reproducing device of the headphone-type is connected and means for terminating the variation in the frequency response of the reproduced signal by the frequency characteristic compensating circuit when the headphone is connected to the auxiliary output terminal.

United States'Patent [191 Olrshima STEREOPHONIC SIGNAL REPRODUCING APPARATUS [75] Inventor: Syunji Ohshima, Kanagawa, Japan [73] Assignee: Sony Corporation, Tokyo, Japan [22] Filed: Oct. 11, 1973 [21] Appl. N0.: 405,426

[30] Foreign Application Priority Data Oct. 13, 1972 Japan 47-118493 [52] U.S. Cl. 179/1 G, 179/15 BT [51] llnt. Cl. H04h 5/00 [58] Field of Search 179/1 G, 15 BO, 1 PC [56] References Cited UNITED STATES PATENTS 3,094,584 6/1963 Dow 179/1 G 3,160,706 12/1964 Bauer et al. 179/1 G 3,164,676 l/l965 Brunner .f. 179/1 G 3,329,773 7/1967 Yoshiaki Ochi 179/15 BT 3,413,417 11/1968 Cornell 179]] PC 3,737,581 6/1973 Horigome 179/15 BT Primary Examiner-Kathleen H. Claffy Assistant Examiner-Tommy P. Chin Attorney, Agent, or FirmLewis H. Eslinger [451 Nov. 19, 1974 [5 7] ABSTRACT In a stereophonic signal reproducing apparatus, there are provided left and right loudspeaker systems for reproducing sounds in response to oppositely phased difference signals (L-R) and (R-L) that are supplied thereto, a central loudspeaker system disposed between the left and right loudspeaker systems for reproducing sounds in response to a sum signal (L+R) supplied thereto. and a frequency characteristic compensating circuit operable upon the oppositely phased dif ference signals to vary the frequency response of a signal reproduced from the difference signals such that the reproduced signal strength is relatively high in a low frequency band but relatively low in a high frequency band as compared to the frequency dependent signal strength of the sum signal (L+R). The stereophonic signal reproducing apparatus further has an auxiliary output terminal to which a sound reproducing device of the headphone-type is connected and means for terminating the variation in the frequency response of the reproduced signal by the frequency characteristic compensating circuit when the head- 13 Claims, 10 Drawing Figures PATEMTL I133 1 91974 SHEET 20F 5 1k FREQ uE/VGWHZ) PATENTL raw 1 91974 SHEET 30F 5 X XXXX i; XXxx fimigmsqwc PATENTL 137 I 91974 sum 5 or s BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates generally to a stereophonic signal reproducing apparatus and more particularly to a stereophonic signal reproducing apparatus in which reproduced sounds from left, right and central speaker systems are acoustically mixed in space in a matrix manner.

2. Description of the Prior Art In a prior art stereophonic signal reproducing apparatus, a difference signal (L-R) between a left signal L and a right signal R and an oppositely phased difference signal (R-L) are respectively applied .to left and right speaker systems to produce sound reproductions, a sum signal (L+R to the left and right signals is applied to a central speaker system to produce sound reproductions. The thus produced sounds are acoustically mixed in space to induce the sensation of sound expansion in the left and right directions. By means of a frequency characteristic compensating circuit, the frequency response of the reproduced sounds from the difference signals (L-R) and (R-L) modified such that signal strength is made high as compared with the signal strength of the sounds reproduced from the sum signal (L+R) in a low frequency band, but low in a high frequency band to thereby reduce noise in the high frequency band. In such a prior art stereophonic signal reproducing apparatus, if the reproduced sound is heard through a headphone connected to an auxiliary output terminal of the apparatus, the matrix of sounds in space cannot of course, be expected. As a result, the different frequency response characteristics of the sounds reproduced from the difference signals (L-R), (R-L) and the sounds reproduced from the sum signal (L+R) due to the frequency characteristic compensation circuit prevent the proper reproduction of the left signal L and the right signal R, which are well separated acoustically.

SUMMARY OF THE INVENTION A stereophonic signal reproducing apparatus according to the present invention acoustically mixes in space difference signals (.L-R), (R-L) and a sum signal (L+R) from speaker systems in a matrix manner, and has a frequency characteristic compensating circuit for varying the frequency response of the difference signals (L-R) and (R-L). In the apparatus, when an auxiliary output terminal is used to derive reproduced sounds thereat, thefrequency characteristic compensating circuit is made ineffective.

Accordingly, it is an object of the invention to provide a stereophonic signal reproducing apparatus that does not exhibit the defects encountered in the prior art.

It is another object of the invention to provide a stereophonic signal reproducing apparatus having an auxiliary output terminal from which an output may be derived without reducing the degree of separation of left and right signals.

It is a further object of the invention to provide a stereophonic signal reproducing apparatus having an auxiliary output terminal adapted to receive a connector device of a headphone or the like, to thereby render a frequency characteristic compensating circuit inoperative.

Additional and other objects, features and advantages of the present invention will become apparent from the following description taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic diagram showing an arrangement of speaker systems used in a stereophonic signal reproducing apparatus according to the present invention which attains an acoustic matrix;

FIG. 2 is a graphical representation of the reproduced frequency responses of the stereophonic signal reproducing apparatus;

FIG. 3 is a block diagram of an example of a stereophonic signal reproducing apparatus;

FIG. 4 is a schematic diagram of a portion of the apparatus depicted in FIG. 3;

FIGS. 5 and 6 are statistical graphs representing the auditory perception derived from the stereophonic signal reproducing apparatus of the present invention and a stereophonic signal reproducing apparatus using another sound matrix;

FIG. 7 is a block diagram of the device used to obtain the auditory perception depicted in FIG. 6;

FIG. 8 is a graphical representation of the frequency characteristics of the device shown in FIG. 7;

FIG. 9 is a schematic diagram of an example of the stereophonic signal reproducing apparatus according to the invention; and

FIG. 10 is a schematic diagram showing another embodiment of a portion of the stereophonic signal reproducing apparatus of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to FIG. 1, there is illustrated a speaker system of a stereophonic signal reproducing apparatus representing the juxtaposition of a listener P and speakers SP and SP disposed at left and right sides with respect to the listener P. A difference signal (L-R) representing the difference between left and right signals L and R and an oppositely phased difference signal (R-L) are applied to the speakers SP and SP 'respectively, to be reproduced as sounds, while a sum signal (L+R) is applied to a central speaker SP to be reproduced. The speakers may be conventional loudspeaker systems, each including one or more sound producing devices. Thus, reproduced sounds are mixed acoustically in space in a matrix manner, so that the listener hears left and right sound images L and R at the positions expanded outwards from the left and right speakers SP and SP respectively. Hence a stereophonic signal reproducing apparatus of a relatively small size, for example a compact, portable type, having left and right speakers and a central speaker in a common speaker enclosure can produce sound with sufficient expansion to impart satisfactory stereophonic acoustic effect. In order to make the expansion of sound as wide as possible and to improve the sound balance, it is desirable to modify the frequency characteristics of the difference signals (LR) and (R-L).which are apt to be enhanced and distorted in a high frequency region as compared with the sum signal (L+R). Preferably, the reproduction response or level of the difference signals (L-R) and (R-L) reproduced from the left and right speakers SP and SP should be relatively large as compared with the reproduction level of the sum signal (L+R) reproduced from the central speaker SP in the low frequency region of the reproduction frequency, but relatively small as compared with the reproduction level of the sum signal (L+R) in the high frequency region. The frequency response characteristic of the difference signals is preferably as shown in FIG. 2 by curve a and the frequency response characteristic of the sum signal is shown by curve b. In FIG. 2, curve shows the phase relationship of the difference signals (L-R) and (R-L) when the phase of the sum signal L+R) is taken as a reference. In this case, the difference signals are delayed from the sum signal, and the peak response thereof, as shown by the curve a is selected in the vicinity of the frequency range of 200 to 400 Hz. As further shown by the curve a, the signal strength of the difference signal is made to be substantially the same as the signal strength of the sum signal near the frequency of 4KI-Iz.

The frequency responses shown in FIG. 2 represent the electrical frequency characteristics that may be imposed in the signal paths of the difference signals (LR) and (R-L) and the sum signal (L+R), respectively. For example, if an FM radio receiver, as illustratively shown in FIG. 3, is considered, a stereophonic composite signal which contains a difference signal (L R), a sum signal (L+R) and a pilot signal obtained through an FM demodulator or discriminator 2 from a conventional tuner 1 including an IF amplifier, is seen to be supplied to input terminals T, and T of a difference signal system 3 and a sum signal system 4, respectively. The difference signal system 3 comprises a subchannel demodulator 3a, a preamplifier 3b including a frequency characteristic compensating circuit and a low frequency output amplifier 3c having an output coupled to left and right speaker systems SP, and SP,,, respectively. The sum signal system 4 comprises a lowpass filter 4a, a preamplifier 4b and a low frequency output amplifier 4c having an output coupled to speaker system SP The aforedescribed curves a and b of FIG. 2 represent the manner in which the electric frequency characteristics of the signals applied to the input terminals T, and T are modified as these signals are transmitted to the outputs of the low frequency amplifiers 3c and 4c, respectively. The frequency characteristic compensating circuit included in the preamplifier 3b of the difference signal system 3 has, for example, an attenuation characteristic of 12dB per octave and attenuates the reproduction response of the difference signal as it reaches a' high frequency range.

FIG. 4 is a schematic illustration of one practical example of the preamplifier 3b including a frequency compensating circuit 10. In this example, an amplified signal is derived through a capacitor C, from the collector electrode of a transistor Q disposed in grounded emitter amplifier configuration and then transmitted to an output terminal T A series connection of a capacitor C and a resistor R, is. connected between the out put terminal T and ground and a capacitor C, is connected in parallel to the resistor R,. Thus, the transmit ted signal, in this instance the difference signal, is enhanced at its low frequency region, and is attenuated as it arrives at its high frequency region in accordance with the frequency dependent impedance characteristic of the compensating circuit 10. Therefore, the frequency response of the difference signal system 3 exhibits the characteristic shown by the curve a in FIG. 2, notwithstanding the presence or absence of the speaker systems connected thereto In a stereophonic signal reproducing apparatus in which the difference signals (L-R) and (R-L) are applied to left and right speaker systems, respectively, to reproduce sounds therefrom, and the sum signal (L+R) is applied to a central speaker system to reproduce sound therefrom, all the reproduced sounds are mixed in space, referred to as the sound space, in a matrix manner. If the level of sound, reproduced in response to the difference signal is selected to be relatively high in the low frequency region but relatively low in the high frequency region as compared with the level of sound reproduced in response to the sum signal, that is, if the difference signal is modified in the manner depicted by curve a of FIG. 2, a clearer stereophonic sensation can be experienced by a listener.

The above phenomenon has been ascertained by a subjective acoustic test. As shown in FIGS. 5 and 6, a speaker enclosure with a lateral width d of about 35cm was disposed opposite to a listener P and spaced therefrom by about 40 to 50cm. Left and right speaker systems SP, and SP,, were mounted in the speaker enclosure at both sides thereof and a central speaker system SP was disposed within in the speaker enclosure and between the speaker systems SP, and SP,,. The signals from which sounds were reproduced were supplied to the enclosed speaker systems by the apparatus illustrated in FIG. 7. As shown, a signal with a single frequency component was generated by a low frequency oscillator 5. A portion of the generated signal was fed through an amplifier 6 to the central speaker system SP and another portion thereof was fed through a conventional phase inverter switching circuit 7 and an amplifier 8 to the left and right speaker systems SP and SP,,. As schematically depicted the left and right speaker systems were connected in opposite polarities. Thus, the signal generated by the oscillator 5 was reproduced as sounds from the left and right speaker systems SP, and SP with opposite polarities, respectively, and sound images sensed by the listener P were measured. FIG. 5 represents the test results when the frequency response of the sound generating signals was selected to be as shown in FIG. 2, while FIG. 6 represents the test results when the frequency response of the sound generating signals was selected to be as shown in FIG. 8. In the latter figure, the reproduction response characteristic a of the difference signal system and the reproduction response characteristic b of the sum signal system may either be equal to each other, or the response a may be selected to be greater than the response 12.

When the'apparatus of FIG. 7 is used to drive the speaker systems, the reproduced sound from one of the left or right speaker systems SP, and SP,, is seen to be substantially the same in level and phase as the reproduced sound from the central speaker system SP Hence, the sound image sensed by the listener is formed at the side of that one of the speaker systems SP, or SP which is then in phase with the central speaker system SP Accordingly, if the left speaker system SP and the central speaker system SP are driven in phase, by way of example, the listener P senses the sound image at the left side, while if the right speaker system SP and the central speaker system SP are driven in phase by the phase inverter switching circuit 7, the listener P senses the sound image atthe right side. The stereophonic perception is seen to be related to the operation of the phase inverter switching circuit.

The marks X in FIGS. 5 and 6 show the positions of the sound images which were sensed by the listeners when each sound image admitting of the illustrated frequencies was formed at the left side, while the marks show the positions of the sound images which were sensed by listeners when each sound image admitting of the illustrated frequencies was formed at the right side. In this test, 20 listeners were selected. Thus, each mark represents the sensed response by a given listener. For example, FIG. represents, by the marks X, at a frequency of 4KI-Iz, the image positions of the re produced sounds sensed by the listeners. One listener sensed the sound at a point 50cm to the left, three listeners sensed the sound at a point 30cm to the left, five listeners sensed the sound at a point cm to the left, and the like. As shown, one listener sensed the 4KI-Iz sound at a point approximately cm to the right. In FIGS. 5 and 6, curves 1 and 1', which are statistical representations of the sensed sound expansion properties, are formed by joining the averages of the positions of the sound images sensed by the respective listeners. As may be apparent from F IGS. 5 and 6, many listeners particularly sense the expansion of sounds in the frequency range of 300Hz to lKHz. Accordingly, sufficient stereophonic sensation is obtained at this frequency range.

As reflected in the measured results shown'in FIGS. 5 and 6, when the reproduced sound frequency exceeds 4KHz, substantially no directional sensation is derived and the sound is sensed as if it originated from the vicinity of the center. However, if the sound frequency by the dotted curve c, and sounds in the range of'about 300I-Iz to 4KHz, which sounds have a greater influence upon the perception of stereophonic sensations, are not reproduced from the left and right speaker systems. Accordingly, sound expansion is not produced. Further, since distortions in the difference signals is likely to appear in the high frequency region, the above phenomenon is even less desired.

Therefore, if the reproduction response from the left and right speaker systems is enhanced in the low frequency region but attenuated in the high frequency region, sufficient sound in the preferred frequency region that produces a stereophonic effect is reproduced.

Thus, the positions of the sound image become clearer,

sound expansion can be sensed and acoustic balance is improved.

In some stereophonic signal apparatus an auxiliary output terminal may be provided for receiving auxiliary sound reproducing devices, such as the conventional headphone of the two-channel type to achieve stereophonic sound reproduction. If the left and right signals L and R are to be provided at the auxiliary output terminal by matrix mixing the difference signals (L R),

(R-L) and the sum signal (L+R) that are normally supplied to the left and right speaker systems and the central speaker system in the usual matrix manner, the left signal L and the right signal R, which are well separated from an auditory point of view in the sound space, here cannot be obtained because of the different frequency characteristics imposed upon the difference signals and I the sum signals by the frequency characteristic comrange is below 4KHz, it is generally perceived by the listeners as stereophonic sounds. Accordingly, if the signal strength of the reproduced sound that is of a frequency higher than 4Kl-Iz is made relatively low as compared with the signal strength of the reproduced sound from the central speaker, and if the signal strength of the reproduced sound that is in the frequency range of 300Hz to 4KHz, which latter frequency range will have greater influence upon the perception of stereophonic sensations, is made relatively higher than that of the sound reproduced by the central speaker, the positions of the left and right sound images are made clear and hence the expansion of sound can be sensed.

In a low frequency sound region, since the wave length of the sound is relatively long, the sounds generated in response to the left and right signals are repro duced in a sound collecting manner with approximately the same phase. Because of this phase relationship, the difference signals (L-R) and (RL) contain almost no low frequency components. However, if. the wave length of sound is short, and even if the distance from the sound source to an object is short, reproduced sounds are obtained in a sufficiently high frequency region where phase differences are produced. As a result,

pensating circuit described above. FIG. 9-shows one embodiment of an improved stereophonic signal reproducing apparatus in accordance with the present invention wherein properly separated left and right signals are supplied to the device connected to an auxiliary output terminal. Like reference numerals are used to represent the same elements as previously shown in FIGS. 3 and 4.

In FIG. 9, input terminals 11 and 12 receive the difference signal (L-R) and the sum signal (L+R) which are applied to the base electrodes of transistors 0 and 0;, respectively. The emitter electrodes of the transistors are grounded to form the preamplifiers 3b and 4b, respectively. The preamplifier 3b is similar to that shown in FIG. 4 and is provided with the frequency characteristic compensating circuit 10. A load resistor R is connected between the collector electrode of the transistor 01 and a power source +8, and a resistor R is connected between the emitter electrode of the transistor Q1 and ground. The junction defined by the capacitors C, and C is grounded through a variable resistor R for sound intensity (volume) adjustment and the movable tap of the variable resistor R is connected to the input of the low frequency amplifier 3c. In the preamplifier 4b, a load resistor R is connected between the collector electrode of the transistor Q 2 and the power source +8, and a resistor R is connected between the emitter electrode of the transistor Q and ground. The collector electrode of the transistor 0 is coupled to amplifier 40 through a capacitor C and a variable resistor R for sound intensity (volume) adjustment. The movable tap of the variable resistor R is connected to the input of the low frequency amplifier 40.

Output transformers 13 and 14 are connected to the outputs of the low frequency amplifiers 3c and 4c, re-

spectively. The transformers 13 and 14 are provided with primary windings 13a, 14a and secondary windings 13b, 14b, respectively. Negative feedback is provided from the secondary windings 13b and 14b to the amplifiers 3c and 40 through resistors R and R respectively. Other feedback circuits may, of course, be used.

The left and right speaker systems SP and SP are connected in parallel with each other to the secondary winding 13b of the transformer 13 through a connecting device, orjack, and a change over switch which will be described below. The left speaker system SP is adapted to be supplied with the difference signal (LR) and the right speaker system SP is adapted to be supplied with the oppositely phased difference signal (RL). A change over switch 16 for switching the polarity of a signal is connected to the right speaker system SP so as to apply a signal thereto having the same phase as that of the signal applied to the left speaker system SP, when a monoaural signal is received. The central speaker system SP is connected to the secondary winding 14b of the transformer 14 through a connecting device, or jack, and a change over switch and is adapted to be supplied with the sum signal (L+R).

A matrix circuit 18 is provided for mixing the difference signals (LR), (RL) and the sum signal (L+R) to produce the left and right signals L and R therefrom. The matrix circuit is illustratively comprised of resistors R R R and R Auxiliary output terminals (left and right output terminals) and t are adapted to be supplied with the left and right signals L and R from the matrix circuit 18, respectively, and comprise a jack J including a change over switch for connecting stereophonic headphones to the signal reproducing apparatus. A circuit 20 for cancelling or counterbalancing the operation of the frequency characteristic compensating circuit on the difference signals (LR) and (RL) is coupled to the jack J. The circuit 20 is here shown as a reverse compensating circuit for cancelling the frequency compensation provided by the frequency characteristic compensating circuit 10 and preferably exhibits a reverse frequency compensation characteristic. In the illustrated embodiment the circuit 20 is comprised of a capacitor C connected in series to the parallel combination of capacitor C and resistor R,,,. A common terminal is included in the jack J, the common terminal being grounded.

One end 130 of the secondary winding l3b"of the transformer 13 is grounded through a resistor R The midtap of the secondary winding 13b also is grounded. The left output terminal t of the jack J is connected through the resistor R of the matrix circuit 18 to the one end 13c of secondary winding 13b of the transformer 13. The secondary winding 14b of the transformer 14 includes one end 146 connected to the resistors R and another end 14d connected to ground. The right output terminal t;, of the jack J is connected through the resistor R of the matrix circuit 18 to the other end 13d of secondary winding 13b of the transformer 13, and through the resistor R to the one end 140 of secondary winding 14b of the transformer 14.

The jack J, further includes left and right switches SW and SW respectively, each of which comprises a movable contact a and fixed contacts b and c. The movable contacts a of the switches SW and SW are normally in contact with the contacts b thereof but are positioned to contact the other fixed contacts c in response to the force imparted to the movable contacts by the left and right output terminals and 1 through the insulator blocks 22, respectively, when. a plug of a stereophonic headphone (not shown) is inserted into the jack J. It is appreciated that the insertion of a plug tends to separate the output terminals t and t;;.

The movable contact a of the left switch SW is connected to the end of secondary winding 13b of the transformer 13, the fixed contact b of the switch SW is connected in common to the left and right speaker systems SP and SP and the fixed contact 0 of switch SW is electrically isolated. The movable contact a of the right switch SW is connected to ground, the fixed contact b of switch SW is connected through the coil of the central speaker system SP to the end 14c of secondary winding 14b of the transformer 14, and the fixed contact c of switch SW is coupled to the emitter electrode of the transistor Q of the preamplifier 3b through the reverse compensation circuit 20. A complete circuit is established by the additional connections of the coils of the left and right speaker systems SP and SP to the end 13 d of secondary winding 13b of the transformer 13.

It may be appreciated that the frequency response of the difference signals (LR) and (RL) is varied by the compensating circuit 10 so as to exhibit the characteristic as shown in FIG. 2 by the curve a and the phase relationship of the difference signals admits of the characteristic represented by the curve (1) in FIG. 2 with respect to the sum signal (L+R). However, the frequency characteristic reverse compensating circuit 20 reversely compensates for the frequency compensation of the difference signals (LR) and (RL) to thus cancel the effects of circuit 10 whereby the frequency response of the difference signals is now similar to that of the sum signal (L+R) as shown by curve b in FIG. 2 and the phase relationship of the difference signals is the same as that of the sum signal (L+R).

The operation of the stereophonic signal reproducing apparatus shown in FIG. 9 will now be described. Let it be assumed that the jack J is not provided with a plug and, therefore, the movable contacts a of switches SW and SW are in contact with their respective fixed contacts b. Accordingly, the difference signal (LR) is transmitted from the end 130 of secondary winding 13b, through contacts a and b of switch SW through speaker system SP to coil end 13d. Similarly, the difference signal (RL) is transmitted from the end 13d of secondary winding 13b, through the change over switch 16 to the right speaker system SP and through contacts b and a of switch SW to coil end 130. Also, the sum signal (L-l-R) is transmitted from the end of secondary winding 14b through the central speaker system SP through contacts b and a of switch SW 2 to ground. In addition, the reverse compensating circuit 20 is connected to contact 0 of switch SW which is here isolated, thus rendering circuit 20 inoperative. Accordingly, the speaker systems provide sound-matrix stereophonic signal reproduction.

When stereophonic headphone plug is inserted into the jack J, the output terminals t and 13 are forced further apart to urge the movable contacts a of the switches SW and SW into contact with the fixed contacts 0, respectively, thereby disengaging their respective fixed contacts b. As a result, the left and right speaker systems SP and SP are disconnected from the transformer 13 and the central speaker system SP is disconnected from the transformer 14. Additionally, the reverse compensating circuit 20 is connected between the emitter electrode of the transistor 0, of the preamplifier 3b and ground by contacts a and c'of switch SW which are 'now closed. Thus, the frequency response characteristic of the output signal from the preamplifier 3b, i.e., the difference signal (LR), is no longer modified and thus exhibits a flat frequency characteristic and also admits of the same phase as that of the sum signal (L+R). It is seen that the effects of compensating circuit 10 are now balanced, or counteracted, by the reverse compensating circuit 20. The difference signal (LR) provided at the end 130 of secondary winding 13b of the transformer 13 and the sum signal (L+R) provided at the end 14c of the secondary winding 14b of the transformer 14 are attenuated by the resistors R and R respectively, and mixed in a matrix manner by the matrix circuit 18 with the result that the left signal L is obtained at the left output terminal Similarly, the difference signal (R-L) provided at the other end 13d of the secondary winding 13!; of the transformer 13 and the sum signal (L+R) provided at the end 14c of the secondary winding 14b of the transformer 14 are attenuated by the resistors R and R respectively, andmixed in a matrix manner by the matrix circuit 18 with the result that the right signal R is obtained at the right output terminal The thus obtained left and right signals L and R are transmitted to the stereophonic headphones where a stereophonic signal with uniform frequency and phase is reproduced.

In the illustrated embodiment, the reverse compensating circuit 20 is comprised of the capacitor C connected in series to the parallel combination of capacitor C and resistor R However a reverse compensating circuit 20' may alternatively consist of only a capacitor C as illustrated in FIG. 9, which may be operatively connected to the emitter electrode of the transistor 0,. Further, the foregoing description has providedfor the termination of the frequency characteristic compensating operation on the difference signals (L-R) and (R-L) by employing a reverse compensating circuit 20 or 20'. In another embodiment it is contemplated that the compensating operation may be terminated by the provision of an ON-OFF switch in the frequency characteristic compensating circuit 10.

Referring to FIG. 10, an ON-OFF switch SW may be provided in the jack J, the ON-OFF switch being actuable (for example, to the OFF state) by inserting a plug connected to the headphones (not shown) into the jack J to permit the plug to be connected to the auxiliary output terminals t and i In this manner, the opening of the illustrated switch SW disconnects the frequency characteristic compensating circuit from the output of the transistor 0,.

The foregoing has described exemplary embodiments that may be varied and modified without changing the basic teachings of the present invention. For example, the sound generating signals may be derived from a received radio signal, prerecorded signals such as a phonograph record, magnetic tape, or the like. Further-' more, the frequency at which the difference signals and the sum signals, exhibit substantially equal signal strengths need not be limited to 4KHz as depicted in FIG. 2. It has been found that equal signal strengths maybe provided in the frequency range of SKI-I2 to 6KHz.

As noted hereinabove, the left, right and central loudspeaker systems may each comprise one or more loudspeakers and driving coils therefor. The sound regions of plural speakers are, of course, selected in the conventional manner well known to acoustic engineers or the like.

Therefore, the foregoing and various other changes and modifications in form and details may be made without departing from the spirit and scope of the invention. Consequently, it is intended that the appended claims be interpreted as including all such changes and modifications.

What is claimed is:

l. A stereophonic signal reproducing apparatus comprising:

a. left and right speaker systems and a central speaker system;

b. means for applying a difference signal (LR) proportional to the difference between left and right signals L and R and an oppositely phased difference signal (R-L) to said left and right speaker systems and for applying a sum signal (L+R) to said central speaker system to thereby produce sounds from said left, right and central speaker systems, the produced sounds being mixed acoustically in space in a matrix manner;

c. frequency characteristic compensating means for varying the reproduction frequency response of said difference signals (LR) and (R-L) such that the level of said difference signals is relatively high in a low frequency band but relatively small in a high frequency band as compared with the level of said sum signal (L+R);

d. a matrix circuit coupled to said means for applying for mixing said difference signals (LR), (R-L) and said sum signal (L+R) in a matrix manner to produce left and right signals L and R, respectively;

e. auxiliary output terminal means coupled to said matrix circuit and adapted to be supplied with said left and right signals L and R; and

f, means operably coupled to said auxiliary output terminal for terminating the variation in the reproduction frequency response of said difference signals.

2. A stereophonic signal reproducing apparatus as claimed in claim 1 wherein said auxiliary output terminal means includes a jack and a switch operable in response to the insertion of connecting means into said jack; and wherein said means for terminating is activated by the operation of said switch.

3. A stereophonic signal reproducing apparatus as claimed in claim 1 wherein said means for terminating comprises a reverse compensating circuit for cancelling the variation in the reproduction frequency response of said difference signals produced by the frequency characteristic compensating means.

4. A stereophonic signal reproducing apparatus as claimed in claim 3 wherein said reverse compensating circuit comprises a series connection of two capacitors and a resistor connected in parallel to one of said two capacitors.

5. A stereophonic signal reproducing apparatus as claimed in claim 3 wherein said reverse compensating circuit is formed of a capacitor.

6. A stereophonic signal reproducing apparatus as claimed in claim 1 wherein said means for terminating is operable to disconnect said frequency characteristic compensating circuit means.

7. A stereophonic signal reproducing apparatus as claimed in claim 1 wherein said matrix circuit and said means for terminating are rendered operable when an output is derived from said auxiliary output terminal means.

8. A stereophonic signal reproducing apparatus as claimed in claim 1 wherein said matrix circuit means and said means for terminating are rendered operable and said speaker systems are disconnected, all at the sametime, when an output is derived from said auxiliary output terminal means.

9. A stereophonic signal reproducing apparatus as claimed in claim 1 wherein said means for applying a difference signal and an oppositely phased difference signal includes an amplifier and said frequency characteristic compensating means is coupled to the output of said amplifier.

10. A stereophonic signal reproducing apparatus as claimed in claim 9 wherein said auxiliary output terminal means includes a jack adapted to receive a plug connected to headphone means and further including switch means operable between first and second sets of contacts in response to the reception of said plug by said jack.

11. A stereophonic signal reproducing apparatus as claimed in claim 10 wherein said means for applying a sum signal includes a further amplifier and wherein said left and right speaker systems are coupled to said amplifier and said central speaker system is coupled to said further amplifier by said first set of contacts of said switch means such that said respective speaker systems are disconnected from said respective amplifiers when said switch means operates in response to the reception of a plug by said jack.

12. A stereophonic signal reproducing apparatus as claimed in claim 11 wherein said means for terminating comprises a reverse compensating circuit connected to one of said second sets of contacts for being operably coupled to said amplifier when said switch means operates in response to the reception of a plug by said jack, to thereby cancel the variation in the reproduction frequency response of said difference signals produced by said frequency characteristic compensating means.

13. A stereophonic signal reproducing apparatus as claimed in claim 11 wherein said means for terminating comprises a normally closed switch for coupling said frequency characteristic compensating means to said amplifier, said normally closed switch being opened in response to the reception of a plug by said jack. 

1. A stereophonic signal reproducing apparatus comprising: a. left and right speaker systems and a central speaker system; b. means for applying a difference signal (L-R) proportional to the difference between left and right signals L and R and an oppositely phased difference signal (R-L) to said left and right speaker systems and for applying a Sum signal (L+R) to said central speaker system to thereby produce sounds from said left, right and central speaker systems, the produced sounds being mixed acoustically in space in a matrix manner; c. frequency characteristic compensating means for varying the reproduction frequency response of said difference signals (LR) and (R-L) such that the level of said difference signals is relatively high in a low frequency band but relatively small in a high frequency band as compared with the level of said sum signal (L+R); d. a matrix circuit coupled to said means for applying for mixing said difference signals (L-R), (R-L) and said sum signal (L+R) in a matrix manner to produce left and right signals L and R, respectively; e. auxiliary output terminal means coupled to said matrix circuit and adapted to be supplied with said left and right signals L and R; and f. means operably coupled to said auxiliary output terminal for terminating the variation in the reproduction frequency response of said difference signals.
 2. A stereophonic signal reproducing apparatus as claimed in claim 1 wherein said auxiliary output terminal means includes a jack and a switch operable in response to the insertion of connecting means into said jack; and wherein said means for terminating is activated by the operation of said switch.
 3. A stereophonic signal reproducing apparatus as claimed in claim 1 wherein said means for terminating comprises a reverse compensating circuit for cancelling the variation in the reproduction frequency response of said difference signals produced by the frequency characteristic compensating means.
 4. A stereophonic signal reproducing apparatus as claimed in claim 3 wherein said reverse compensating circuit comprises a series connection of two capacitors and a resistor connected in parallel to one of said two capacitors.
 5. A stereophonic signal reproducing apparatus as claimed in claim 3 wherein said reverse compensating circuit is formed of a capacitor.
 6. A stereophonic signal reproducing apparatus as claimed in claim 1 wherein said means for terminating is operable to disconnect said frequency characteristic compensating circuit means.
 7. A stereophonic signal reproducing apparatus as claimed in claim 1 wherein said matrix circuit and said means for terminating are rendered operable when an output is derived from said auxiliary output terminal means.
 8. A stereophonic signal reproducing apparatus as claimed in claim 1 wherein said matrix circuit means and said means for terminating are rendered operable and said speaker systems are disconnected, all at the sametime, when an output is derived from said auxiliary output terminal means.
 9. A stereophonic signal reproducing apparatus as claimed in claim 1 wherein said means for applying a difference signal and an oppositely phased difference signal includes an amplifier and said frequency characteristic compensating means is coupled to the output of said amplifier.
 10. A stereophonic signal reproducing apparatus as claimed in claim 9 wherein said auxiliary output terminal means includes a jack adapted to receive a plug connected to headphone means and further including switch means operable between first and second sets of contacts in response to the reception of said plug by said jack.
 11. A stereophonic signal reproducing apparatus as claimed in claim 10 wherein said means for applying a sum signal includes a further amplifier and wherein said left and right speaker systems are coupled to said amplifier and said central speaker system is coupled to said further amplifier by said first set of contacts of said switch means such that said respective speaker systems are disconnected from said respective amplifiers when said switch means operates in response to the reception of a plug by said jack.
 12. A stereophonic signal reproducing apparatus as claimed in claim 11 wherein said means for terminating comprises a reverse compensatIng circuit connected to one of said second sets of contacts for being operably coupled to said amplifier when said switch means operates in response to the reception of a plug by said jack, to thereby cancel the variation in the reproduction frequency response of said difference signals produced by said frequency characteristic compensating means.
 13. A stereophonic signal reproducing apparatus as claimed in claim 11 wherein said means for terminating comprises a normally closed switch for coupling said frequency characteristic compensating means to said amplifier, said normally closed switch being opened in response to the reception of a plug by said jack. 